Controlling the texture and crystallinity of evaporated lead phthalocyanine thin films for near-infrared sensitive solar cells

To achieve organic solar cells with a broadened spectral absorption, we aim to promote the growth of the near-infrared (NIR)-active polymorph of lead phthalocyanine (PbPc) on a relevant electrode for solar cell applications. We studied the effect of different substrate modification layers on PbPc thin film structure as a function of thickness and deposition rate (r_dep). We characterized crystallinity and orientation by grazing incidence X-ray diffraction (GIXD) and in situ X-ray reflectivity (XRR) and correlated these data to the performance of bilayer solar cells. When deposited onto a self-assembled monolayer (SAM) or a molybdenum oxide (MoO₃) buffer layer, the crystallinity of the PbPc films improves with thickness. The transition from a partially crystalline layer close to the substrate to a more crystalline film with a higher content of the NIR-active phase is enhanced at low r_dep, thereby leading to solar cells that exhibit a higher maximum in short circuit current density (J_SC) for thinner donor layers. The insertion of a CuI layer induces the formation of strongly textured, crystalline PbPc layers with a vertically homogeneous structure. Solar cells based on these templated donor layers show a variation of J_SC with thickness that is independent of r_dep. Consequently, without decreasing r_dep we could achieve J_SC = 10 mA/cm ², yielding a bilayer solar cell with a peak external quantum efficiency (EQE) of 35% at 900 nm, and an overall power conversion efficiency (PCE) of 2.9%.